Have I got a doozy for you with my 2002 Oldsmobile Intrigue with the 3.5 V6 “Shortstar” engine. I must warn you that this has not only myself stumped but also m friend, the professional diesel mechanic. It all started a couple weeks ago on a chilly Michigan spring day when it was necessary to run the heat. I was idling in a parking lot with the heat running and before long I notice that it’s spitting out cold air. Odd, I thought, thinking that the engine needed to warm up a bit. And warm up it did! Far higher than my previous experience with normal operating temperature. I quickly shut it off and gave it a chance to cool down. When it did I started it back up hoping the problem would go away, like rebooting a computer (kidding) and sure enough, no heat and I watched the temp climb a little past normal temp. I started to drive to the mechanic keeping a sharp eye on the temp gauge and to my surprise when I started moving the heat came on and the engine temp came down!
I know what you’re thinking. Bad thermostat, or bad pump. Me too. That’s the reason for the excess of information. You should also know that I’ve known that I had a leaky crossover for some time and Ive kept my eye on the coolant level in the reservoir. Oddly enough, it never creeped too low…until we went to drain the system to replace the thermostat, then I realized the upper radiator hose was completely empty! We were able to leave some fluid in the reservoir when we replaced the thermostat and once replaced and hooked back up to thermostat, it drained into the system. Eureka! We refilled the fluid and fired her up. Here’s where it gets weird. We had a heat gun and determined that the new thermostat is doing what it should. All hoses were the proper temp, all fans working. We even checked to make sure the heater core inlet and outlet had a wide temp range, and they did. In the vehicle however, the vents were reading at 36 degrees with the heat on high! A/C clutch was also engaged. We pulled the relay to disengage the A/C clutch (because it’s still cold here and I didn’t want it to freeze up) and I still get cold air…until we start driving. As soon as we did the temp rose. Heat gun read it at over 100 degrees, until we came to a stop sign, then it dropped a full 25 degrees. My ONLY thought is air in the system, but this thing is supposed to be self-bleeding with the reservoir at the highest point (which it’s not, the heater core hoses are) So if it IS an air pocket, how do I bleed it, and also, WHAT THE HECK IS GOING ON?! We’re stumped.
I think you should just stay with the air in the system idea. Are you sure it doesn’t have bleeder screws? All of the 3.4Ls do. I would deal with that leaky crossover. Its not the most common way for a leak to occur, but cooling system leaks can suck air in addition to or instead of letting out coolant. So if that leak is sucking air its not implausible that it first settles in the heater core, and then gets pushed along once you drive.
You also mentioned the water pump but only talked about working on the thermostat. It doesn’t sound like that’s been ruled out. I’d also worry about a restricted radiator. Do you still have this thing on Dexcool? That stuff can make a mess, especially if the cooling system is compromised by leaks/air entering.
Look for the bleed screws on the hoses.
By “heatgun”, do you mean IR thermometer? It has a trigger and emits a red beam when you pull the trigger.
You have to start with fixing all known issues. You have a leak in the crossover, you should have fixed this before jumping on the thermostat. When you fix this, the problem may go away, but if it doesn’t, then you can troubleshoot the problem.
If you still have a leak in the crossover the system will draw air in through that instead of coolant from the expansion tank. When you get enough air in that causes a low temp reading because the sensor is exposed to air, Then when you drive the car some of the air gets circulated giving you a temp. reading and heat. That works until you get moe air, the symptoms come back and the coolant level is so low that the car overheats, but the gauge might not tell you.
Dinger, my sympathies to you. I had a 2001 Intrigue with only 95K miles on it when it started developing this problem 2 years ago, running hot and losing coolant. Systematically, and one at a time I had all radiator/cooling system parts replaced starting with the obvious and easiest to replace items (spending about 4K total). The head gaskets were replaced several times (as another person here suggested on my post last summer) as when it runs hot these are compromised. Even after all of this effort, the car continued to have this problem that worsened over time. At the end, I had to add 1.5 gallons of coolant (just water at this point) every day before driving and the car’s range before overheating got progressively shorter. Recently, I sold the car for parts (the exterior and interior were in great shape) and when the 3.5 liter engine was removed and disassembled most of the inner block coolant channels were corroded and clogging. The mechanic was familiar with this problem and would have recommended having a rebuilt engine installed instead of all the work I did. BTW - the bleeder valve (shaped like a wing nut) is on the upper left side (facing the car) of the radiator. This was used to remove air from the coolant system which was another suggestion that didn’t work. If this matches your problem, you may want to cut your losses now.
Here’s the quickest way to purge any ait from the cooling system.
Get the engine up to operating temperature. With the engine idling, loosen the upper radiator hose clamp. Take a small flat bladded screwdriver and insert it between the the upper radiator hose and the radiator hose neck. Allow the engine to idle until nothing but coolant comes out of the hose. Remove the screwdriver and retighten the hose clamp.
@Dinger to be brutally honest, I believe the heater core should be flushed, from both sides
I’ve fixed MANY heater problems this way, including some where the laser thermometer seemed to indicate that the heater core couldn’t be the problem.
Can you see the heater hoses entering the firewall? if so can you feel them or shoot them with the IR gun and see what the delta T is between the inlet and outlet? they should be almost the same or with in 10-15 degrees of each other and close to the engines temp.
Like everyone else stated, fix the cross over first, then make sure all of the air is out of the system.